The chemical equation for the combustion of hydrogen is 2H2 + O2 → 2H2O. During hydrogen combustion, the heat from burning makes hydrogen chemically react with oxygen gas, O2, in the air to form water.
In this reaction, two molecules of hydrogen gas, H2, react with one molecule of O2 to form two molecules of water, H2O. A significant amount of energy, in the form of heat, is required to start the reaction. Due to the presence of extreme heat, the water created by the reaction is not a liquid but is gaseous water vapor. The reaction also generates large amounts of heat, which is produced by the breaking of the chemical bonds connecting the gases.